**4. Conclusions**

During the last decade, the interest in the recovery of bioactive polyphenols from olive leaves has increased in the industry and the scientific community. Oleuropein and luteolin-7-glucoside extraction from olive leaves could add value to this by-product of the olive oil industry if they are recovered by applying efficient extraction technologies. The present work investigated the influence of UAE experimental conditions (increment of temperature and amplitude) on the extraction yield of the two main olive leaf polyphenols, showing a significant increase of oleuropein and luteolin-7-glucoside contents in the extracts (6.6% and 37.9%, respectively) in comparison to Soxhlet conventional extraction. The extraction samples are heated using ultrasound, so that the greater the amplitude, the higher the heating rate. The optimal results were obtained at low amplitudes and high temperatures. The highest extraction efficiency and antioxidant capacity were obtained under optimal increment of temperature and amplitude conditions (40 ◦C and 30% amplitude). Under these conditions, the simultaneous optimization of oleuropein and luteolin-7-glucoside provided a concentration of oleuropein of 69.91 g/kg whilst the concentration of luteolin-7-glucoside was 1.82 g/kg. The results demonstrated that the UAE is quicker and more efficient than conventional Soxhlet method, providing olive leaves extracts with higher amounts of oleuropein and luteolin-7-glucoside, but significantly reducing the extraction time (around only 15 min are required versus 4 h of the traditional extraction) and energy costs. This work can help to approach new studies on the UAE process on a larger scale for later industrial valorization of olive leaves. This process could provide large enough amounts of the bioactive compounds studied which will allow more analyses of their biological activities and improvement in the understanding of their properties.

**Author Contributions:** Conceptualization, A.L.M., F.E. and M.M.; methodology, A.L.M. and M.d.M.C.; software, M.M.; writing—original draft preparation, A.L.M and M.d.M.C.; writing—review and editing, A.L.M., F.E. and M.M.; supervision, E.C.; project administration, F.E.; funding acquisition, A.L.M., F.E. and I.R.

**Funding:** This research was funded by University of Jaén (Acción 6) and Ministerio de Ciencia, Innovación y Universidades of Spanish Government (Project code: ENE2017-85819-C2-1-R).

**Acknowledgments:** The authors want to thank the technical and human support provided by CICT of the University of Jaén (UJA, MINECO, Junta de Andalucía, FEDER).

**Conflicts of Interest:** The authors declare no conflict of interest.
